(S)-3-(indol-3-ylmethyl)-6,7,8,8a-tetrahydropyrrolo[1,2-a]pyrazin-1-one + dimethylallyl diphosphate = (S)-3-{[2-(1,1-dimethylallyl)-indol-3-yl]methyl}-6,7,8,8a-tetrahydropyrrolo[1,2-a]pyrazin-1-one + diphosphate — reaction